The Ocean’s Low Oxygen Areas Shrink During Earlier Warm Periods. So why are we growing? – Is it good?

Essay by Eric Worrall

Yet another mystery to settlement science.

Climate change and ocean oxygen: Shrinking oxygen-poor regions during past warm periods, scientists discover

via Princeton University
AUGUST 31, 2022

Over the past 50 years, hypoxic areas in the open ocean have increased. Scientists attribute this development to rising global temperatures: There is less dissolved oxygen in warmer water, and layers of tropical oceans may become more stratified.

But now, contrary to many’s expectations, an international team of scientists led by researchers from the Max Planck Institute for Chemistry and Princeton University has found that hypoxic regions have shrunk. for long warm periods in the past.

“We did not expect such a clear effect,” said Alexandra Auderset, first author of the new paper in the journal. Nature and is currently a postdoctoral fellow at Princeton University. She led the study with Alfredo Martínez-García at the Max Planck Institute for Chemistry in Mainz, as part of a long-term collaboration with Daniel Sigman’s group at Princeton University.

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However, it remains unclear what this means for the current expansion of hypoxic open ocean areas, Auderset said. “Unfortunately, we still don’t know if our findings on shrinking marine hypoxia zones are applicable for the coming decades or just for the longer term,” she said. “This is because we have to address whether short-term or long-term processes are responsible for the change.”

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Read more: https://phys.org/news/2022-08-climate-ocean-oxygen-oxygen-poor-zones.html

Summary of the study;

Enhanced ocean oxygenation during the warm Paleozoic period

Published: August 31, 2022
Alexandra Auderset, Simone Moretti, Björn Taphorn, Pia-Rebecca Ebner, Emma Kast, Xingchen T. Wang, Ralf Schiebel, Daniel M. Sigman, Gerald H. Haug & Alfredo Martínez-García

abstract

Dissolved oxygen (O₂) is essential for most ocean ecosystems, providing energy for respiration and facilitating carbon and nutrient cycling. Oxygen measurements have been interpreted to show that the ocean’s hypoxia zones (ODZs) are expanding as the earth warms.^first,2. However, the models provide an unclear picture of future ODZ change in both the short and long term.^3,4,5,6. Paleontological records can help uncover the range of possible ODZ changes in modern warmer periods. Here we use the foraminifera-bonded nitrogen isotope (N) to show that water column denitrification in the eastern tropical North Pacific was significantly reduced during the Middle Miocene Optimal (MMCO). ) and Early Eocene Climate Optimization (EECO). Because denitrification is restricted to oxygen-poor waters, our results indicate that, during these two Cenozoic periods of sustained warmth, the ODZs shrank, not expanded. The ODZ contraction may result from a decrease in bioenergized yield in the tropical Pacific, which would reduce oxygen demand in the subsurface. Alternatively, increased deep-water ventilation of the Southern Ocean may have weakened the ocean’s ‘biocarbon pump’, which would have increased the amount of oxygen in the deep ocean. The mechanics at play will determine whether ODZ contractions occur with warming or take centuries or millennia to develop. Thus, although our results from the Protozoa do not necessarily apply to the near future, they may imply that global warming may eventually cause a contraction of the ODZ.

Read more: https://www.nature.com/articles/s41586-022-05017-0

Personally, I was surprised by this discovery. I accept the hypothesis that emissions from warmer water will slightly lower oxygen levels in the ocean, although given the abundance of ocean life in earlier warm periods, I don’t think it will. a problem.

Obviously it would be funny to assume that this unexpected discovery is evidence that the oceans are cooling, and this could actually be the explanation – sea levels have dropped 6ft since the end of the Holocene Optimum 6000 years ago. before. It is possible that our modern warm period is too short a blip to influence ocean oxygen changes due to our long-term Holocene cooling trends.

But I accept Princeton’s explanation, which we don’t know.